Galaxy Physics

Mark Whittle

University of Virginia

Outline

1. Galaxy basics : scales, components, dynamics

2. Galaxy interactions & star formation

3. Nuclear black holes & activity

4. (Formation of galaxies, clusters, & LSS)

Aim to highlight relevant physics and recent developments

1. Galaxy Basics

• Scales & constituents

• Components & their morphology

• Internal dynamics

Galaxies are huge

• Solar sys = salt crystal– Galaxy = Sydney

• Very empty– Sun size = virus (micron)

– @ sun : spacing = 1m

– @ nucleus : spacing = 1cm

• Collisionless – Average 2-body scattering ~ 1 arcsecond

– Significant after 10^4 orbits = 100 x age of universe

– Stars see a smooth potential

Constituents

• Dark matter– Dominates on largest scales

– Non-baryonic & collisionless

• Stars – About 10% of total mass

– Dominates luminous part

• Gas – About 10% of star mass

– Collisional lose energy by radiation

– Can settle to bottom of potential and make stars• Disk plane : gas creates disk stars (“cold” with small scale height)

• Nucleus/bulge : generates deep & steep potentials

– Historically ALL stars formed from gas, so behaviour important

Galaxy Components

• Nucleus

• Bulge

• Disk

• Halo

Bulges & disks

• Radically different components

• Ratio spread ( E – S0 – Sa – Sb – Sc – Sd )

• Concentrations differ (compact vs extended)

• Dynamics differ (dispersion vs rotation)

• Different histories (earlier vs later)

Disks : Spiral Structure• Disk stars are on nearly circular orbits

– Circular orbit, radius R, angular frequency omega

– Small radial kick oscillation, frequency kappa

– View as retrograde epicycle superposed on circle

• Usually, kappa = 1 – 2 omega orbits not closed– (Keplerian exception : kappa = omega ellipse with GC @ focus)

– Near the sun : omega/kappa = 27/37 km/s/kpc

• Consider frame rotating at omega – kappa/2 – orbit closes and is ellipse with GC at centre

• Consider many such orbits, with PA varying with R

• Depending on the phase one gets bars or spirals• These are kinematic density waves • They are patterns resulting from orbit crowding• They are generated by :

– Tides from passing neighbour

– Bars and/or oval distortions

– They can even self-generate (QSSS density wave)

– Amplify when pass through centre (swing amplification)

• Gas response is severe shocks star formation

Disk & Bulge Dynamics

• Both are self gravitating systems– Disks are rotationally supported (dynamically cold)

– Bulges are dispersion supported (dynamically hot)

– Two extremes along a continuum

– Rotation asymmetric drift dispersion

• What does all this mean ?– Consider circular orbit, radius R speed Vc

– Small radial kick radial oscillation (epicycle)

– Orbit speeds : V<Vc outside R, V>Vc inside R

• Now consider an ensemble of such orbits

GC

morestars

fewer stars

<V> less than Vc

• Consider stars in rectangle – Mean velocity mean rotation rate (<V>)

– Variation about mean dispersion (sig)

• In general <V> less than Vc

• For larger radial perturbations, <V> drops and sig increases– Vc^2 ~ <V>^2 + sig^2

• This is called asymmetric drift (clearly seen in MW stars)

• Extreme cases : – Cold disks <V> = Vc and sig = 0 pure rotation

– Hot bulges <V> = 0 and sig ~ Vc pure dispersion

• More complete analysis considers :– Distribution function = f(v,r)d^3v d^3r

• This satisfies a continuity equation (stars conserved)– The collisionless Boltzmann equation

• Difficult to solve, so consider average quantities– <Vr>, <sig>, n (density), etc

– This gives the Jean’s Equation (in spherical coordinates)

– Which mirrors the equation of hydrostatic support : dp/dr + anisotropic correction + centrifugal correction = Fgrav

• Hence, we speak of stellar hydrodynamics

2. Interactions & Mergers

• Generate bulges (spiral + spiral = elliptical)

• Gas goes to the centre (loses AM)

• Intense star formation (starbursts)

• Supernova driven superwinds

• Chemical pollution of environment

• Cosmic star formation history

Spiral mergers can make Ellipticals

During interactions : – Gas loses angular momentum

– Falls to the centre

– Deepens the potential

– Forms stars in starburst

stars

Gas/SFR

Enhanced star formation

Blowout : environmental pollution via superwinds

Cosmic star formation history

HDF

3. Nuclear Black Holes & Activity

• Difficulties & methods

• Example #1 : the milky way

• Other examples : gas, stars, masers

• Black hole demographics – links to the bulge

• Black hole accretion : nuclear activity

• Cosmic evolution – ties to mergers and SF

Example #1 : the milky way

Other galaxies : methods

• Need tracer of near-nuclear velocity field– Defines potential M(r)

– If more than M(stars) dark mass present

• Obvious tracers : stars and/or gas– Doppler velocities (proper motions)

– Note : both rotation &/or dispersion present

– Use Jeans Equation M(r)

Pure rotation – gas or cold star disk

isotropic dispersion

anisotropic dispersion

* Gas &/or star disks are best

* Bulge stars are poor, unless isotropy known

Activity : accretion onto the BH

• Gravitational energy near Rs ~ 50% rest mass• Accretion requires AM loss : MHD torques• Energy liberated as photons & bulk flow

– Luminous across the EM spectrum

– Powerful outflows, some at relativistic speeds

• Accretion associated with galaxy interactions• ? Black hole formation associated with mergers ?• Quasar history linked to merger/SFR history

Quasar and Galaxy Evolution

• Quasar/Starburst/Galaxy evolution related ?• Major mergers

– Extreme star formation rates

– Elliptical/bulge formation

– BH formation and feeding = QSO

• Evidence – Comparable luminosity in QSO and starburst

– Most luminous nearby mergers are also QSOs

– QSO evolution loosely follows SFR history

• Currently speculative – active area of research

4. Galaxy Formation Theory• Mature subject – semi-analytic & numerical• Two important observational constraints

– Galaxy luminosity function (many small, few large)– Galaxy large scale structure (clusters, walls, voids)

• Start with uniform DM (+ baryon) distribution– Add perturbations matched to CMB– Embed in comoving expansion & add gravity

• Follow growth of perturbations : linear – non-linear– Semi-analytic useful but limited– Numerical follows full non-linear development + mergers– Baryon physics recently included (pressure, cooling, SF,

…)